Recently, semiconductor devices using metal oxide semiconductor thin films have attracted attention. For example, as discussed in Japanese Patent Application Laid-Open No. 2000-150900, the above-mentioned thin films are characterized by being able to be formed at low temperatures, and having a large optical band gap so as to be transparent to visible light. Thus, a flexible and transparent thin film transistor (TFT) can be formed on plastic substrates, film substrates, and the like.
Generally, the oxide semiconductor films used as TFT active layers have hitherto been formed in an atmosphere with oxygen gas introduced thereinto for the purpose of controlling the electrical characteristics of the films. For example, WO2005-088726 A1 discusses a thin film transistor (TFT) in which an n-type oxide semiconductor including In—Ga—Zn—O is used for the channel layer, and ITO is used for the source and drain electrodes; in this TFT, by controlling the oxygen partial pressure of the film formation atmosphere in which the In—Ga—Zn—O film is formed, the carrier concentration has been controlled and a high carrier mobility has been obtained.
Additionally, Japanese Patent Application Laid-Open Nos. 2006-165529 and 2006-165532 discuss techniques for annealing in an oxidative atmosphere the active layers in the semiconductor devices using as the active layers thereof the oxide semiconductor films.
However, the oxide semiconductor films discussed in the above-mentioned patent documents are narrow in the oxygen partial pressure range (optimal oxygen partial pressure) under which the In—Ga—Zn—O films usable as the TFT active layers can be obtained, for example, as shown in WO2005-088726 A1. Additionally, there has been a problem such that the value of the optimal oxygen partial pressure is varied depending on the film formation conditions. Further, the present inventors have found that when In—Ga—Zn—O films are formed by a sputtering method in an atmosphere with oxygen introduced thereinto, the electrical characteristics of the formed In—Ga—Zn—O films are varied depending on the distance from the target. This is conceivably because the dissociation degree of the oxygen molecules present in the vapor phase is largely varied depending on the distance from the target.
Additionally, a TFT adopting as the channel layer thereof an In—Ga—Zn—O film fabricated in the film formation atmosphere with oxygen introduced thereinto can hardly attain such transistor characteristics as anticipated from the physical characteristics intrinsically possessed by the In—Ga—Zn—O film. Specifically, it is necessary to further improve the TFT characteristics such as the subthreshold characteristics in the subthreshold region and the field-effect mobility.
Further, when a DC stress of a voltage of 12 V is applied for 400 seconds to each of the gate electrode and the source and drain electrodes, the characteristic variations such as the turn-on characteristic degradation and threshold voltage variation are caused, and thus the DC stress affects the TFT. The main cause for the degradation of the TFT characteristics is conceivably ascribable to the damage to the semiconductor film effected by the oxygen ions generated from the oxygen introduced into the film formation atmosphere. Specifically, when the film formation is conducted by using a sputtering method, conceivably the oxygen introduced into the atmosphere increases the amount of the oxygen negative ions generated on the target surface, and consequently, the amount of the high energy oxygen negative ions incident onto the substrate is increased to cause the film quality degradation.
Additionally, the present inventors have fabricated and studied a thin film transistor (TFT) adopting as the channel layer thereof an In—Ga—Zn—O amorphous oxide thin film, and consequently, have found that the TFT is atmosphere sensitive in such a way that the characteristics of the TFT are varied by the atmospheres during operation and storage.
The fabricated device was placed in a vacuum chamber, and the electrical conductivity was measured while the chamber was being evacuated, and consequently a phenomenon was found that the measured value decreased gradually with decreasing pressure. When measured in a usual atmosphere, the measured value of the electrical conductivity was found to be stable.
Such an atmosphere-induced variation of the electrical conduction is also seen, for example, for ZnO, indium tin oxide (ITO) and the like; such a variation is conceivably due to the adsorption and desorption, to and from the conductive oxide, of the water and other gaseous molecules in the atmosphere. Thus, there has been caused a problem that an atmospheric variation induces variation in electrical conductivity to destabilize the TFT operation and consequently no reliability as a device can be attained.
Additionally, the present inventors have found a problem that in the formation of an oxide insulator by sputtering on an In—Ga—Zn—O amorphous oxide thin film, the film formation in an atmosphere without oxygen contained therein causes a large variation of the electrical characteristics of the oxide semiconductor so as to prohibit the attainment of satisfactory TFT characteristics.
The present invention solves the above-mentioned problems, and an object of the present invention is to provide a production method of an oxide thin film transistor having a wide process margin, excellent characteristics and high reliability.
Additionally, another object of the present invention is to provide a production method of a thin film transistor having high reliability attained by overcoming the TFT characteristic instability ascribable to the atmosphere variation.
Additionally, yet another object of the present invention is to provide a production method of a thin film transistor in which when an oxide insulator is formed on an In—Ga—Zn—O amorphous oxide thin film, the electrical characteristics of the oxide semiconductor are not varied.